In Latin, Y was named I graeca. This was pronounced as E grecka, since the classical Greek sound /y/, similar to modern German ü or French u, was not a native sound for Latin speakers, and the letter was initially only used to spell foreign words. In Romance languages, this history has led to the standard modern name of the letter: In Galician i grego, in Catalan i grega, in French and Romanian i grec - all meaning "Greek I". The names igrek in Polish and i gờ-rét in Vietnamese are both phonetic borrowings of the French name. In Dutch, both Griekse ij and i-grec are used. In Spanish, Y is also called i griega, however, in the twentieth century, the shorter name ye was proposed and was officially recognized as its name in 2010 by the Real Academia Española, although its original name is still accepted.[3] The original Greek name upsilon has also been adapted into several modern languages; in German, for example, it is called Ypsilon, and in Italian the name is ípsilon or ípsilo. In Portuguese, both names are used (ípsilon and i grego).[4]

The oldest direct ancestor of English letter Y was the Semitic letter waw, from which also come F, U, V, and W. See F for details. The Greek and Latin alphabets developed from the Phoenician form of this early alphabet. In Modern English, there is also some historical influence from the old English letter yogh (Ȝȝ), which developed from Semitic gimel, as shown below.

Y first appeared as the Greek letter upsilon. The Romans borrowed a small form of upsilon as the single letter V, representing both the vowel sound /u/ and the consonant /w/. (In modern ways of writing Latin, V is typically written as U, for a vowel, or V for the consonant.) However, this first loaning of upsilon into Latin is not the source of Modern English Y.

The usage of the capital form of upsilon, 'Y' as opposed to U, V, or W, dates back to the Latin of the first century BC, when upsilon was introduced a second time, this time with its "foot" to distinguish it. It was used to transcribe loanwords from the prestigious Attic dialect of Greek, which had the non-Latin sound /y/, as found in modern French cru (raw), or German grün (green). Because it was not a native sound of Latin, it was usually pronounced /u/ or /i/. The latter pronunciation was the most common in the Classical period and was used by most people except Greek educated ones.[citation needed]

The letter was also used for other languages with a /y/ sound. Some words of Italic origin were re-spelled with a 'y': Latin silva ('forest') was commonly spelled sylva, in analogy with the Greek cognate and synonym ὕλη.[5]

In Old English there was a native /y/ sound, and so both Latin U and Y were adapted for use. By the time of Middle English, /y/ had lost its roundedness and became identical to I (/iː/ and /ɪ/). Therefore, many words that originally had I were spelled with Y, and vice versa. (Some dialects, however, retained the sound /y/ and spelled it U, following French usage.)[citation needed]

Likewise, Modern English vocalic Y is pronounced identically to the letter I. But Modern English uses it in only certain places, unlike Middle and early Modern English. It has three uses: for upsilon in Greek loan-words (system: Greek σύστημα), at the end of a word (rye, city; compare cities, where S is final), and before vowel endings (dy-ing, justify-ing).

As a consonant in English, Y is normally a palatal approximant, /j/ (year, German Jahr). This is possibly influenced by the Middle English letter yogh (Ȝȝ), which represented /j/. (Yogh's other sound, /ɣ/, came to be written gh in Middle English, and although the sound is no longer pronounced in standard modern English silent gh is common in many words where this sound was once present, such as through and caught, and in some cases an /f/ sound has resulted in modern English, as in rough or trough.)

When printing was introduced to Great Britain, Caxton and other English printers used Y in place of Þ (thorn: Modern English th), which did not exist in continental typefaces. From this convention comes the spelling of the as ye in the mock archaism Ye Olde Shoppe. But in spite of the spelling, pronunciation was the same as for modern the (stressed /ðiː/, unstressed /ðə/). Ye (/jiː/) is purely a modern spelling pronunciation.[6]

The letter Y was originally established as a vowel. In the standard English alphabet, the letter Y is traditionally regarded as a consonant, but a survey of almost any English text will show that Y more commonly functions as a vowel.

Y has the sound values /y/ or /ʏ/ in the Scandinavian languages and in German. It can never be a consonant (except for loanwords), but can in German appear in diphthongs, as in the name Meyer, where it serves as a variant of ⟨i⟩. In Norwegian it forms part of the diphthong ⟨øy⟩, which in Swedish is spelled ⟨öj⟩ and ⟨øj⟩ (formerly ⟨øi⟩) in Danish.

In Dutch, Y appears only in loanwords and names and usually represents /i/. It may sometimes be left out of the Dutch alphabet and replaced with the ligature IJ. In addition, the Y is occasionally used instead of an IJ, albeit very rarely. In the Afrikaans language, a descendant of Dutch, Y denotes the diphthong [ɛi], which may derive from the IJ ligature.

The Icelandic writing system uses y for /ɪ/ and ý for /i/. In Faroese, Y is always pronounced /i/. In both languages, it can also form part of diphthongs such as ⟨ey⟩ (in both languages) and ⟨oy⟩ (Faroese only).

In French orthography y is identical to i in pronunciation. It is pronounced as [i] when a vowel (as in the words "cycle", "y") and as [j] as a consonant (as in "yeux", "voyez"). It alternates orthographically with i in the conjugations of some verbs, indicating a [j] sound.

Appearing alone as a word, the letter Y is a grammatical conjunction with the meaning "and" in Spanish and is pronounced /i/. In Spanish family names, y can separate the father's surname from the mother's surname as in Santiago Ramón y Cajal; another example is Maturin y Domanova, from the Aubrey-Maturin series. Catalan names use i for this. As a consonant y represents [ʝ] in Spanish. When coming before the sound /i/, the conjunction Y is replaced with E: español e inglés. This is to avoid pronouncing /i/ twice.

The letter Y is called i/y griega, literally meaning "Greek I", after the Greek letter ypsilon, or ye.

It is mostly used in loanwords from English, Japanese, Spanish, Russian and Hebrew. Loanwords in general, primarily gallicisms in both varieties, are more common in Brazilian Portuguese than in European Portuguese. It was always common for Brazilians to stylize Tupi-influenced names of their children with the letter (which is present in most romanizations of Old Tupi) e.g. Guaracy, Jandyra, Mayara – though placenames and loanwords derived from Indigenous origins had the letter substituted for ⟨i⟩ over time e.g. Nictheroy became Niterói.

To a minor degree (often stigmatized as a signal of the lower classes) it is also true for common Western/Christian in Brazil, together with those of immigrant communities, although the practice is not possible in Portugal where names are required to follow official spelling conventions (see more at Portuguese name).

Usual pronunciations are /i/, [j], [ɪ] and /ɨ/ (the two latter ones are inexistent in European and Brazilian Portuguese varieties respectively, being both substituted by /i/ in other dialects). The letters ⟨i⟩ and ⟨y⟩ are regarded as phonemically not dissimilar, though the first corresponds to a vowel and the latter to a consonant, and both can correspond to a semivowel depending on its place in a word.

In Lithuanian Y is the 15th letter and is a vowel. It is called the long i and is pronounced /iː/ like in English see.

When used as a vowel in Vietnamese, the letter y represents the sound /i/; when it is a monophthong, it is functionally equivalent to the Vietnamese letter i. Thus, Mỹ Lai does not rhyme but mỳ Lee does. There have been efforts to replace all such uses with i altogether, but they have been largely unsuccessful. As a consonant, it represents the palatal approximant. The capital letter Y is also used in Vietnamese as a given name.

It is indicative of the rarity of front rounded vowels that [y] is the rarest sound represented in the IPA by a letter of the Latin alphabet, being cross-linguistically less than half as frequent as [q] or [c] and only about a quarter as frequent as [x].[citation needed]

The IPA symbol [j] ("jod") represents the sound of the English letter ⟨y⟩ in the word yes.